1. Field of the Invention
The present invention relates to a construction of a door beam of a motor vehicle, and particularly to a door beam of a motor vehicle provided in a vehicle door and fixed at opposite ends thereof to door side panels.
2. Description of the Prior Art
To prevent a vehicle door from being distorted due to a side collision of a motor vehicle so as to secure an occupant in safety, a door beam, in general, has been provided in the door to increase the rigidity of the door.
The door beam of the prior art is of such an arrangement that a section modulus for the beam is determined in consideration of a buckling strength of the section at the center of the door beam for bearing the maximum bending moment acting thereon through the total longitudinal length of the door beam, and the door beam has, over the total length thereof, the contour of the section substantially corresponding to the aforesaid section modulus. However, in the door beam of the prior art as described above, since the section over the total longitudinal length thereof is set up based on the section modulus bearable of the maximum bending moment, the section moduluses at opposite sides from the center of the door beam become unnecessarily large. Therefore, the door beam is increased in weight to an unnecessary extent and it becomes difficult dimension-wise to be disposed within a small space defined by a window pane and a door inner or outer panel in the door. Furthermore, in the door beam of the prior art as described above, since the maximum bending moment acting on the door beam increases in proportion to the total longitudinal length of the door beam, the maximum bending moment acting on the center of the door beam, in case of a long door in the longiutdinal direction, is increased in value, thus presenting disadvantageous conditions in weight and dimensions.
Furthermore, as with the door beam of the prior art, where it is impossible to increase the sectional area for increasing the section modulus due to the restriction in the space for providing the door beam in the door, there have been adopted a construction having a closed type section or a clad type section by adding a reinforcing beam at the center or over the total longitudinal length of the door beam. In that case, however, the door beam becomes complex in construction and the weight thereof increases. Furthermore, as in a door beam 1 shown in FIG. 1 as another example of the door beam of the prior art, in which the door beam has a cross-section of angular wave shape to increase the section modulus, a drawing width B against a unit drawing depth D becomes large due to a restriction in the work of deep drawing into the angular wave shape, whereby the total height H becomes large. As a result, the door beam becomes heavy in weight and the space required for providing the door beam becomes large.